This is an epidemiologic study attempting to test hypotheses relating to variation in menstrual cycle patterns, the hormonal and endocrine correlates of these patterns, and the subsequent relationship to the incidence of breast cancer. The study is conducted in two populations. The first is a case-control study of menstrual cycle patterns, family history, disease history, exogenous hormone and medication consumption, and related variables in patients with breast cancer and two selected control groups. The second portion of the study concerns the analysis of a longitudinal study of concurrently recorded menstrual cycle patterns in a group of women who are being followed for breast cancer incidence. In this latter population we are undergoing three activities: 1) we are asking these women to recall already recorded menstrual and reproductive events to determine the validity of the recall of such events. 2) We have looked at menstrual cycle patterns in women with the presence or absence of various breast cancer risk factors to see if such patterns are different. 3) We are determining the breast cancer incident events in this population and looking retrospectively at cycle patterns in those who do or do not develop breast cancer. Combining these epidemiologic data with hormonal determinations associated with various menstrual patterns from clinical research, we are elaborating hypotheses about the menstrual antecedents of breast cancer and attempting to develop menstrual pattern risk factors which may be useful in identifying the high risk breast cancer patient.